It is often desired to position a rotatable element, such as a motor shaft to a preselected position. For example, it may be desired to continually urge an oscillatable shaft toward a preselected, or rest position. Thus, when the oscillating torque is removed from the shaft, the shaft is returned to such preselected, or rest, position. If the shaft carries a mirror or the like, the mirror is returned to a particular angular position, which is determined by the rest position of the shaft. Often, such a shaft is driven by a small, bi-directional, low-inertia stepper motor.
Small stepper motors are readily available and may be purchased, for example, capable of providing incremental shaft rotation of 7.5 degrees and multiples thereof, depending on the wiring and the drive signal. Their consumed power, for example, may be on the order of 2 watts, continuous duty, developing approximately 2 ounce-inches of torque. Such motor might be driven by a 7 to 12 volt, a-c signal, which would cause the motor shaft to angularly rotate, or oscillate, as the alternating signal reverse polarity. That is, the motor shaft reverses direction as the electrical signal reverses.
For example, a laser scanner (used, for example, for light display purposes) may utilize such a bi-directional stepper motor, which is excited by a-c electrical signals to rotate its shaft through a small angle, say, plus and minus 15 degrees. The shaft carries a small mirror which receives and reflects a laser beam. When the shaft is angularly rotated, or oscillated, by the motor, it causes the reflected laser beam to sweep through an angle. When the signals to the motor are weakened or removed, it is desired that the shaft return smoothly from its angular rotation to a rest position. To achieve a smooth, harmonic-like motion, the shaft is ordinarily spring-loaded. Such spring-loading returns the shaft to a position approximately central of its sweep. It is appreciated, however, that the system could be driven to scan more to one side of its rest position than another. In such a case, the shaft's rest position may not be at the center of the scan.
The device of the invention provides smooth return of the shaft to its rest position by magnetic repulsion, rather than by use of a spring, as is customary in such devices.
The magnetic repulsion device of the invention may be used on any angularly rotatable shaft (or on structure attached to such shaft) which requires a smooth return to a rest position. The motion of the shaft, upon application of electrical signals to the driving motor, or prime mover, is harmonic motion or akin thereto.
The shaft may be used, in other instances, to carry oscillating, light chopper means, light-switching means, or other devices. The device of the invention be used not only for shafts carrying radiation transmitting devices, such as the mirrors for laser beam scanners, but may also be used in connection with shafts driving radiation receiving devices, such as scanners (including mirrors, for example,) receiving laser beams, microwaves, light, or other electromagnetic or acoustic waves.
For example, a sensor may, by means of the scanner herein, be made to scan an area for incoming infrared, visible light, invisible light, or other radiation.
In summary, the device may be one which either transmits or receives radiation.
In any event, what is desired is the capability to urge a driven shaft toward a preselected position. When driving power, or torque, is removed from the shaft, it is desired to cause a smooth return of the driven shaft to the preselected position. In the case of a laser scanner or light chopper used for light display purposes, this results in smoothly-changing figures and light displays. At times, abrupt changes in the displays may be desirable and may be achieved by the driving motor or by light choppers or other means. The device of the invention is particularly suited to operating under such circumstances.
Commonly, the return of a shaft or lever arm to its rest position in mechanical and electro-mechanical devices is achieved by a spring. In this invention the magnets take the place of springs. The magnets are superior to springs in length of life and in ease of installation and adjustment. In structures which oscillate rapidly over long periods of time, such as a laser scanner, or a light chopper, the spring is under continually changing stress, which ultimately causes fatigue, degradation and failure of the spring to return the shaft to its rest position. There may even be a certain amount of slop, in the mechanical spring construction, causing failure of the system to respond consistently. Further, spring mechanisms suffer from hysteresis, both at high and low levels of tension, torsion or compression.
Further, it is extremely difficult to assemble and adjust a spring-centering means so that it provides equal return torque from angular excursions of a shaft in both directions. Bi-direction spring loading is not easily obtained, changed or adjusted.
On the other hand, the device of the invention, a magnetic centering means is extraordinarily simple, does not suffer from fatigue and experiences little or no hysteresis effects. Permanent magnets are readily available with a magnetic permanence that far exceeds the life of a spring.
The magnetic device of the invention is readily adjustable as to strength and position. It is difficult to change the spring constant of a spring. In the case of permanent magnets, a simple adjustment of the position of the magnets relative to each other, changes the effective spring constant, (or, more precisely, the magnetic coercion). Further, diminutive, ceramic magnets (sintered from barium and iron oxide powders) of various strengths and sizes are readily available. They are almost impossible to demagnetize except in extreme heat. Small, disk-shaped magnets which would be suitable for use in connection with the 2-watt stepper motor mentioned above, would likely be on the order of 0.125 inches thick. They would have a diameter of, say, 0.3 of an inch and the north and south poles would be on opposite faces of the disk. Such magnets, when properly oriented with respect to each other, would have a magnetic coercion (repulsion) between them, when in close proximity, that would not be overcome by the torque of the stepper motor. Thus, the magnets would not, in the usual case, strike each other.
It is also noted that, in the device of the invention, as the deflection increases, the restoring force increases nonlinearly. On the other hand, in the case of a spring, the restoring force increases linearly, unless the spring is specially constructed to have a variable spring constant.
Another feature of the magnetic centering device, in one embodiment, is that the weight of one of the magnets, which is attached to the shaft, hangs as a pendulum, to aid in returning the shaft to a rest position. This is not so in the case of a spring because it must be anchored at both ends in order to be effective.